Conventional ground-based radars are generally incapable of resolving multiple targets that are spatially separated by less than the beam width of the radar signal at a given target range, because the radar signal reflected from closely spaced targets appears as a single target reflection. First one, then another of the targets may produce the stronger return pulse, so that the weaker reflected signal is buried and the azimuth and range of the target appears to vary in a random manner. In contrast, airborne synthetic aperture radar (SAR) systems can resolve such multiple, closely spaced targets in azimuth and range as the antenna on the aircraft moves along a predefined vector. The SAR system processes the resulting radar Doppler signal to separately resolve the positions of each target. However, this technique cannot be implemented by ground-based radar systems that are generally fixed in position.
In U.S. Pat. No. 3,849,779, multiple targets are resolved within one radar beam width by detection and analysis of either a phase-modulated component or an amplitude-modulated component of the radar signal that is modulated at twice the scan rate of the radar. The phase-modulated component is preferably employed if a speed gate/Doppler frequency tracker in the radar system is engaged; otherwise, the amplitude-modulated component is used. This system appears to be limited to resolving only two closely spaced targets.
Another multiple target radar tracking system is disclosed in U.S. Pat. No. 3,323,128, which teaches that at least two sets of orthogonally crossed line array antennas can be used to improve the resolution of a radar system. One of the sets of antennas is rotatable through a known angle relative to the other. The multiple output signals developed by the antennas are input to a computer so that the phase relationship of the radar signals reflected from plural targets can be analyzed to correlate the target locations. Apparently, the number of targets that can be simultaneously identified is limited to the number of sets of antenna arrays.
In a paper entitled "Feasibility of a Synthetic Aperture Radar with Rotating Antennas (ROSAR)," delivered before the 19th European Microwave Conference in London, U.K., in September 1989, Helmut Klausing described a ROSAR (Rotor-SAR) airborne radar system having improved resolution. In the ROSAR system, radar antennas are mounted at the tips of helicopter rotor blades, which thus form a rotating turntable. The resulting radar signal is used to produce a synthetic aperture image. However, in the ROSAR system, the nature of the rotating turntable does not permit compensation for the beam on-target amplitude and polarization changes that occur with rotation of the blades. Consequently, the system is spatially coherent for only about 60 degrees of turntable (rotor blade) rotation. The ROSAR system can resolve multiple fixed targets in a single field of view, but not moving targets. In addition, the ROSAR system does not have scan-to-scan coherence, i.e., the polarization, amplitude, and phase of each target are not maintained over successive rotations of the rotor blades. As a result, the effective resolution of the radar system is reduced.
In consideration of the resolution limitations of conventional radar systems, including the prior art radar systems discussed above, it is an object of the present invention to continuously coherently resolve a plurality of targets spaced apart by less than a radar signal beam width. It is a further object to resolve more than two such targets, with respect to range and azimuth, with a radar signal transmitted from either a generally fixed or moving position. These and other objects and advantages of the present invention will be apparent from the attached drawings and the Description of the Preferred Embodiments that follow.